Hardening of aluminum



Patented Oct. 17,1933

HARDENING OF ALUMINUM Robert S. Archer, Milwaukee, Wis, ass ignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application March 4, 1931 Serial No; 520,155

Claims. (Cl. 148- 7) The invention, which relates to aluminum and aluminum base alloys, is directed to a'proce'ss for producing aluminum or aluminum alloy products having a surface of a hardness substantially 5 greater than the hardness of the interior metal.

For purposes of simplicity, the word aluminum as used herein and in the appended claims is defined to include not only pure metallic alumi num" but also aluminum in all commercial grades 10 of purity as well as aluminum base. alloys contain-' of hardening may become quite brittle. In many instances where hardness is the desideratum, it is only surface hardness that is required and it becomes immaterial if the hardness of the interior of the metal does not equal that of the surface. Infact, it is as often advantageous that themetal present a hard exterior surrounding a more yielding and ductile interior. Such soft core articles are characterized by high surface resistance to wear, yet withstand the sudden shocks or repeated stresses encountered in service. For instance, at the present time, a large number of pistons for internal combustion engines are made of aluminum, the number annually sold amounting into the millions, but there is; nevertheless, a widespread demand for a more wear-resistant aluminum piston which will retain all the desired characteristics of the aluminum now used but which will present a harder and, consequently, more wear-resistant surface;

Because of the demand for such articles, "I have been led to devise my present invention which consists in packing the aluminum article to be treated in a mass of comminuted magnesium metal in such manner and under such conditions that the surface'of the article and the particles of the magnesium powder are in close and intimatecontact, and thereafter subjecting the thus packed aluminum article and comminuted magnesium metal to a thermal treatment for such lengths of time and at such temperatures as will produce the results hereinafter described;

Preliminary to packing the aluminum article in the magnesium metal, the surface of the artical is preferably treated so that it willbe substantially free from dirt, oil, oxides, or other extraneous material. I have found that exceptionally good results are obtained if the cleaning operation employed is one which roughens thesurface of the article slightly. To accomplish such a surface cleaning, the aluminum article may be rubbed with an abrasive or treated with a solution of an acid or alkali, or a combined treatment may bev used. These are usual methods of cleaning aluminum surfaces and similar and other well known methods may likewise be used with satisfactory results. The comminuted magnesium metal in which the aluminum article is then packed should preferably be of a pure grade and .in a comparatively fine state of subdivision. One form of comminuted'magnesiu-m metal which I have found efiective is sometimes known as cutter material and consistsof the fine chips and curls of metallic magnesium produced by machining a magnesium ingot. Magnesium powder as it is sold in commercial form and other similar forms of comminuted magnesium metal are also satisfactory. 1

Thealuminum article may be packed in the comminuted magnesium metal in any suitable receptacle which fits conveniently in the heating furnace which is to be employed. Because, however, of the propensity of magnesium: metal to oxidize readily and even ignite at high temperatures, I have found it necessary to exclude during the heating process any substantial quantity of air. It is possible to .obtain satisfactory results by placing the comminuted magnesium metal and the aluminum article packed therein in an airtight receptacle but I prefer to accomplish this purpose during heating by introducing into the heating furnace an atmosphere of some gas which is inert'to magnesium, such as carbon dioxide, which will preclude the necessity of the use of airtight receptacles.

The aluminum article packed in the comminuted magnesium metal is then, in accordance with my invention, heated to a temperature sufficient to' cause diffusion of the magnesium into the surface of the aluminum article. The tem-, perature at which such diffusion becomes rapid enough to be commercially practicable is usually above about 250 centigrade when a close and intimate contact betweenthe magnesium metal and the surface of the aluminum article exists. For rapiddifiusion, however, I find it preferable to heat the packed article at temperatures in the neighborhood of about 420? centigrade to about 460 centigrade, but since the eutectic mixture formed by the aluminum and magnesium or by r the diffusion of the magnesium into the aluminum melts at about 450 centigrade, it is not desirable to substantially exceed that temperature although a small amount of incipient fusion upon the surface of the aluminum article will not be deleterious in effect. If, however, there is a possibility of incipient fusion at the points of contact between the comminuted magnesium metal and the 4 hardness.

The time for which the heating should be carried out depends upon the temperature, a higher temperature requiring a shorter time to produce a given result. The effect of my novel treatment is to produce on the aluminum article a hardened surface, which surface is composed of an alloy of aluminum and magnesium, and the depth of which depends upon and increases with the temperature employed and the time of treatment.

The existence, extent, and depth of the hardened surface may be readily determined by microscopic examination or by testing'the surface with a file. Aluminum articles treated in accordance with my invention can be machined, filed, or cut more easily than untreated articles, and are also capable of taking a high polish. I

The article produced in accordance with my invention is characterized by the fact that the surface formed on the aluminum article is not readily distinguishable chemically, mechanically, or visually from the metal of the core. The surface is not magnesium per se but is an alloy of aluminum and magnesium formed by the diffusion of the magnesium into the outer surface of the treated aluminum article.

Exemplary of the practice of my invention is an instance where an aluminum base alloy containing 13 per cent of siliconwas packed in comminuted magnesium metal and heated for 20 hours in an air-tight receptacle at a temperature of about 440 centigrade.- The article after treatment had a surface hardness of 26 as measured by the sclerosoope, whilethe interior metal or core had a hardness of only 9. An aluminum base alloy containing 4 per cent of copper and inherently harder than the aforementioned silicon alloy had, after a similar treatment, a surface hardness of 29 and a core hardness of 21. Thermal treatments subsequently applied to articles which have been treated in accordance with my invention for the purpose of modifying or producing certain properties of the aluminum may change the actual hardness of both the surface and interior of the metal but a relative hardness differential produced as the result of my novel treatment will usually be retained.

Having thus described and explained my invention and the method'in which it may be practiced,

I claim: V

1. A method of increasing the hardness of an aluminum article at or near the surface, comprising packing the aluminum article in close and intimate contact .with comminuted magnesium metal and heating the packed article at temperatures above about 250 centigrade and below about 460 centigrade.

2. A method of increasing the hardness of an aluminum article at or near the surface, .comprising placing the aluminum article in close and intimate contact with a comminuted material comprising magnesium, and heating the article and comminuted material to a temperature above about 250 centigrade and below about 460 centigrade while excluding substantial amounts of air therefrom.

3. An aluminum alloy article having a relatively soft interior core and a relatively hard exterior surface, the said exterior surface consisting of an alloy of aluminum and magnesium formed bydiffusion.

4. A method of increasing the hardness ofan aluminum article at or near the surface, comprising placing the aluminum article'in close and intimate contact with comminuted material comprising magnesium metal and magnesium oxide and heating the article and comminuted material to a temperature above about 250 centi 115 grade and below about 460 centigrade.

5. A method of increasing the hardness of an aluminum article at or near the surface, comprising placing the aluminum article in close and intimate contact with a comminuted material comprising magnesium, and heating the article and comminuted material to a temperature above about 250 centigrade and below about 400 centtgradewhile substantially excluding air therefrom until magnesiumdifluses into said article andalloys therewith and increases the surface hardness thereof, substantially as described. I

ROBERT S. a 

